Data Sheet
June 1999
ORCA Series 3C and 3T FPGAs
almost a full cycle. This is shown in Figure 48A. The
amount of delay that is being compensated for, plus
clock setup time and some margin, is the amount less
than one full clock cycle that the output clock is delayed
from the input clock.
Programmable Clock Manager (PCM)
(continued)
PCM Applications
The applications discussed below are only a small
sampling of the possible uses for the PCM. Check the
Lucent Technologies ORCA FPGA Internet website
(listed at the end of this data sheet) for additional appli-
cation notes.
In some systems, it is desirable to operate logic from
several clocks that operate at different phases. This
technique is often used in microprocessor-based sys-
tems to transfer and process data synchronously
between functional areas, but without incurring exces-
sive delays. Figure 48B shows an input clock and an
output clock operating 180° out of phase. It also shows
a version of the input clock that was shifted approxi-
mately 180° using logic gates to create an inverter.
Note that the inverted clock is really shifted more than
180° due to the propagation delay of the inverter. The
PCM output clock does not suffer from this delay. Addi-
tionally, the 180° shifted PCM output could be shifted
by some smaller amount to effect an early 180° shifted
clock that also accounts for loading effects.
Clock Phase Adjustment
The PCM may be used to adjust the phase of the input
clock. The result is an output clock which has its active
edge either preceding or following the active edge of
the input clock. Clock phase adjustment is accom-
plished in DLL mode by delaying the clock. This is dis-
cussed in the Delay-Locked Loop (DLL) Mode section.
Examples of using the delayed clock as an early or late
phase-adjusted clock are outlined in the following para-
graphs.
In terms of degrees of phase shift, the phase of a clock
is adjustable in DLL mode with resolution relative to the
delay increment (see Table 27):
An output clock that precedes the input clock can be
used to compensate for clock delay that is largely due
to excessive loading. The preceding output clock is
really not early relative to the input clock, but is delayed
<
Delay 16
Phase Adjustment = (Delay)* 11.25,
Phase Adjustment = ((Delay)* 11.25) – 360, Delay > 16
CLOCK DELAY AND SETUP
BEING COMPENSATED
DLL DELAY
INPUT CLOCK
OUTPUT CLOCK
A. Generating an Early Clock
UNINTENDED PHASE
SHIFT DUE TO
INVERTER DELAY
DLL DELAY
INPUT CLOCK
PCM OUTPUT CLOCK
INVERTED INPUT CLOCK
B. Multiphase Clock Generation Using the DLL
5-5979(F)
Figure 48. Clock Phase Adjustment Using the PCM
Lucent Technologies Inc.
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